As a general rule, every internal combustion engine, whether it operates in Diesel cycle or in Otto cycle, on 2 strokes or 4 strokes, comprises at least one piston with its respective rings.
The piston ring is the part that performs the function of sealing the space between the engine sleeve and the piston, isolating the combustion chamber from the other internal engine components. This element is arranged radially at the piston top, preventing the combustion gases from escaping out of the combustion chamber toward the crankcase and preventing the engine oil from penetrating the combustion chamber on a reverse direction. There may be more than one piston ring surrounding a single piston, and it is very common to use three rings arranged parallelly at the piston top.
Generally, prior-art piston rings 10′ comprise an opening between two ends 4′, even when they are mounted on the piston and confined by the cylinder, as shown in FIG. 1 of this document. Among other functions, the opening is useful during the installation of the prior-art piston ring 10′ around the piston (not shown in the figures). The opening provides access to a tool configured to open the ring 10′, enlarging its inner diameter beyond the external radial limits of the piston, thus enabling access thereof to the ring groove, that it, the recess in the outer face of the piston where the ring is housed.
In summary, a piston ring should have good sealing capability, and in order to guarantee this characteristic along the useful life of the engine a piston ring should exhibit adequate mechanical stability, breaking strength, and also adequate resistance to wear with respect to its tribological pairs: the cylinder for its contact face and the piston groove for its side contact faces. Excessive wear of the side faces and of the contact face ends up causing degradation of the sealing main function of the combustion chamber and results in abrupt degradation of the functioning conditions of the engine.
Many of the efforts to provide a solution to wear of piston rings have been directed to the development of coatings and surface treatments that impart better resistance to wear, with a direct impact on the durability of the engine.
Moreover, it is also known that the design of a piston ring (that is, its three-dimensional geometric form) can contribute to a longer useful life of the engine by controlling the wear of the piston rings and providing appropriate pressure distribution, thus providing greater homogeneity of the wear along the peripheral surface, or functional surface of the piston ring.
In terms of wear of piston rings, particularly a compression ring (or first-groove ring), the most affected part of the component is the region of ring surface that is in contact with the cylinder. Throughout this contact surface—hereinafter called outer radial face—there is a specific portion that undergoes even greater wear than the average wear which this outer radial face undergoes: the regions close to the ring ends (called critical zones 1 1—see FIG. 4). This increase in wear is due, above all, to three main factors: the process of manufacturing the ring, the mechanical loading of the engine and the thermal conditions imposed to the ring during the operation of the engine.
Considering that at present the engines undergo higher demands in terms of environmental impact and of output, all the engine components should perform their functions, but in a more demanding manner due to either a more severe working condition or a lower level of final damage accumulated on the components after operation, in this context the piston rings are no exception. In this regard, it is necessary to prolong the useful life of the piston ring, more particularly in the end regions. This wear is particularly more marked on heavy duty Diesel engines (HDD).
One of the surface treatment operations used in the prior art to impart greater surface resistance to these parts is the physical vapor deposition process, capable of depositing, for instance, a chrome nitride layer of about 30 microns throughout the peripheral surface that contacts the cylinder, including the end regions 4′ of the ring 10′.
One of the prior-art proposals that try to solve the problem of wear of piston-ring ends is presented in document US 2004056425, which describes a low-cost ring-manufacture method and which provides a countermeasure for the wear of the ends. In this technology, a film is deposited onto the outer surface or slide surface, where the film thickness in the end region (vertex) is larger than in the adjacent region (see FIG. 1). The coating can be applied either by physical vapor deposition (PVD/DFV) or by chemical vapor deposition (CVP/DQV), wherein the end region is exposed longer than the rest of the ring in order to guarantee a larger coating thickness at the ends.
However, it should be noted that the PVD/CVD processes exhibit a great limitation with respect to the final thickness of the coating. Usually, the thickness is not larger than 30 microns, although there are a few cases that make reference to coatings with larger thickness. Anyway, thicker coatings will have a poor performance, since the coating undergoes an increase in tension as a result of the thickness and also loses properties of the material of the layer, which impairs the resistance to wear when one obtains layers with thickness larger than 30 cm. Moreover, although the document mentions a low cost of the process employed, it is known that PVD/CVD processes are quite expensive, and it is natural that the larger the thickness the greater the impact on the final price.
Other attempts were also made to solve the problem of excessive wear in the end region of piston rings. Such solutions can be found in documents JP2000120866, DE102008035148, US2010090416 and US200204107, which mention geometric relations implemented with a view to diminish the contact pressure area in the end portion of the piston ring during the working conditions, these solutions being considered capable of decreasing wear in said end regions of piston rings.
Although this theory seems to have a reasonable technical basis, it is known that such an approach does not solve the problem of high wear at the ring ends. In other words, the solution of exposing the base material of the ring with a PVD/CVD coating is the solution that makes use of the geometric approach.
Thus, the present invention indicates a new direction toward coating the end regions with a layer of a determined thickness deposited onto another.